Luis is 5 times as old as Jessica and is also 16 years older than Jessica. How old is Luis?
Solution: We can use the given information to write down two equations that describe the ages of Luis and Jessica. Let Luis's current age be $l$ and Jessica's current age be $j$ $l = 5j$ $l = j + 16$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $l$ is to solve the second equation for $j$ and substitute that value into the first equation. Solving our second equation for $j$ , we get: $j = l - 16$ . Substituting this into our first equation, we get the equation: $l = 5$ $(l - 16)$ which combines the information about $l$ from both of our original equations. Simplifying the right side of this equation, we get: $l = 5l - 80$ Solving for $l$ , we get: $4 l = 80$ $l = 20$.